Applications for electrophoresis, an analytical technique for separating and identifying biologically important molecules in a sample, include the determination of a sample's homogeneity, the determination of molecular weights of proteins and nucleic acids, the mapping of nucleic acid primary structures, i.e. DNA and RNA sequence analyses, and the definition of phenotypic variance of a protein at the molecular level. Electrophoretic techniques rely on the fact that each molecular species has a unique combination of mass, size, shape, charge, density and sub-unit structure, all of which result in mobility differences responsive to an electric field. Various electrophoretic techniques use one or more of these properties to cause varying degrees of molecular separation via the migration of the molecular species under a constant or varying electric field.
Capillary zone electrophoresis is a technique using a capillary tube which is filled with a conductive fluid, or buffer solution. A small amount of a sample is introduced at one end of the capillary tube, whereafter a high potential difference is applied across the ends of the tube. Differences in the electrophoretic mobilities of different molecules cause the constituents of the sample to emerge separated at the outlet end of the capillary tube. Capillary zone electrophoresis is described in detail in U.S. Pat. No. 4,842,701 to Smith et al.
Typically, the capillary tube is encased within a linear housing, as shown in U.S. Pat. No. 4,705,616 to Andresen et al. Access to the capillary tube through the encasement is difficult, at best. Yet, access is desirable since capillary tubes have a tendency to clog. A clogged capillary tube normally is not repairable and, therefore, must be replaced.
In addition to the need to periodically repair or replace a clogged capillary tube, free access to the tube is desirable because it permits a change of capillary tubes to best fit an application. As noted above, there are a great number of applications for capillary zone electrophoresis. The operational characteristics of the processes vary with the application. Large diameter electrophoresis capillary tubes permit a greater current flow, but the increased current and the greater susceptibility to convection heating translates into a greater concern for the effects of heating than must be faced in use with small diameter capillary tubes. Heat affects and may even destroy the quantitative and qualitative analysis. On the other hand, use of a small diameter capillary tube makes detection of sample constituents more difficult. As the separated molecular constituents of a sample migrate toward the outlet end of the capillary tube, an electropherogram is obtained by employment of an optical detector. Optimally, the electropherogram shows spaced-apart peaks for the individual constituents of the sample. Small diameter capillary tubes are less conducive to such detection. Thus, the operational characteristics of a particular application are a factor in determining the preferred capillary tube diameter for that application. Likewise, the operational characteristics must be considered in any decision as to the length of the capillary tube for a particular application.
It is an object of the present invention to provide an apparatus for electrophoretic separation in which capillary tubes may be selectively changed by a user without a great expense to or mechanical handling by the user. Another object is to provide such an apparatus in a compact manner.